How to add metalness to a specific color on a card
In your image editor select corresponding color on your card and make black and white mask for metal, where white is metallic part.
Invert that mask for roughness map (or make new mask for more control).
Modify your card material in node editor to use principled shader, where base color connects to your original image.
Use your metal mask for metal, rough mask for roughness.


If you want to get more adventurous, you can make a 3rd mask where you set your 'paper' material with fine-grain greyscale noise, and you can apply it with bump map.

Your final shader would look something like this (notice non-color data for masks):



If you find some masks give too strong/weak effect, you can use ColorRamp from Convertor menu for fine-grain control.